1. Field of the Invention
This invention relates to a process for preparing improved polyamides. More particularly, this invention relates to a process for preparing polyamides useful for the formation of films, fibers and other articles and having an improved receptivity or affinity for basic dyestuffs.
2. Description of the Prior Art
Polyamides have been generally prepared by the ring-opening polymerization of .omega.-lactams, or the polycondensation of .omega.-aminocarboxylic acids or diaminodicarboxylates (hereinafter the polymerization reaction and the polycondensation reaction will be designated polymerization for simplicity), and the need for films, fibers and other articles prepared from polyamides have steadily been increased because of their excellent tensile strength and good hand and dye-receptivity or affinity for disperse dyes or acid dyes.
However, one of the disadvantages of polyamides is that these polyamides generally do not possess receptivity or affinity for basic dyes. Recently, polyamides of a high receptivity or affinity for basic dyes having brilliant colors and high dye-ability have been wanted and a method for introducing sulfonate groups into the polymer chain of polyamides has been proposed. For example, U.S. Pat. Nos. 3,039,990 and 3,142,662 disclose compounds having the formulas (II) and (III) ##STR2## wherein M is an alkali metal, X and X' are --COOH, --R'COOH, --COOR", --R'COOR" or --R'NH.sub.2 in which R' represents an alkylene group having 1 to 5 carbon atoms and R" represents an alkyl group having 1 to 5 carbon atoms, and n' is an integer of from 1 to 2. However, when the above compounds are used in the polymerization of polyamides, they are generally bonded to the main chain of a polyamide molecule or the terminal of a polyamide molecule in view of the chemical structure of these compounds and, therefore, a relatively large amount of the compounds should be copolymerized in order to obtain sufficient affinity for basic dyes. Thus, the resulting polyamides using a large amount of these compounds of the formula (II) or (III) generally have a low degree of polymerization and exhibit poor spinnability, especially, poor formation of yarns thereby reducing the fiber properties such as tensile strength and elongation property. These disadvantages ultimately adversely affect the quality of articles produced from such polyamides. Also, when a polyamide-forming compound, for example, a diamine or a dicarboxylic acid is used in a controlled amount by considering the amount of the amido-forming groups in the compound having the formula (II) above, the degree of polymerization in the resulting polyamides can be increased to some extent, but the polyamides obtained in such a case tend to be stained by acid dyes because such polyamides generally contain an approximately equal number of terminal amino groups and carboxyl groups. Further, when these compounds are incorporated into polyamides, the resulting polyamides exhibit an increase in their melt viscosity as a result of the property of the sulfonic acid ion and thus the yield of the polyamide recovered from the polymerization vessel and the handling operability of the polyamides are markedly decreased. In addition, the increase in the melt viscosity adversely affects the spinning yield in the melt spinning of the polyamides, the properties of the resulting fibers, etc.